Ensure that CSSStyleImageValue::imageLayoutSize() doesn't crash if it has no ImageResourceContent

Ensure that CSSStyleImageValue::imageLayoutSize() doesn't crash if
it has no ImageResourceContent when provided an invalid CSSURLImageValue

Also adds a LayoutTest to test this.

BUG=691461

Review-Url: https://codereview.chromium.org/2703393012
Cr-Commit-Position: refs/heads/master@{#453892}
2 files changed